We discuss the surface impedance (Z=R−iX) of a normal-metal/superconductor proximity structure taking into account the spin-dependent potential at the junction interface. Because of the spin-mixing transport at the interface, odd-frequency spin-triplet s-wave Cooper pairs penetrate into the normal metal and cause the anomalous response to electromagnetic fields. At low temperature, the local impedance at a surface of the normal metal shows the nonmonotonic temperature dependence and the anomalous relation R>X. We also discuss a possibility of observing such anomalous impedance in experiments.